There are many applications in which analog signals must be processed rapidly and with high precision. As a general principle it can be stated that the minimum time that the output of an amplifier stage requires to reach a voltage value with a specified degree of accuracy depends on the load capacitance at the output and on the current available for reversing the charge of this load capacitance.
In the prior art the primary means of achieving a fast amplifier is to minimize the load capacitance of the amplifier stage, which often consists of parasitic capacitances and line capacitances. In addition, the maximum output current of the driving stage which is needed for charge reversal of the load capacitance is increased by means of either a constant or signal-dependent increase in the cross current, i.e. of the dc current for a particular operating point, through the output stage.
However, these measures are not unproblematic. Minimization of the load capacitance is often limited due to specification requirements or due to the physical arrangement of the circuit elements and the accompanying line capacitance. Also, increasing the maximum possible output current through a constant increase in the cross current is also limited due to the accompanying increase in the power loss. Furthermore, a signal-dependent increase in the cross current necessitates special circuit measures, which often entail other restrictions. In the case of a pair of push-pull source followers or push-pull emitter followers, in which the respective complementary transistor serves as the load for the other, the output modulation of the whole circuit decreases. Other measures for a signal-dependent increase in the cross current involve more complicated circuitry and may adversely affect the bandwidth and temperature stability of the circuit.
Time-discrete analog signals are special analog signals which have an essentially constant level in any one interval, this level theoretically jumping immediately to another constant level in the following interval. When such time-discrete analog signals are processed by real circuits having an inherently low-pass characteristic, the theoretically infinitely steep flanks at the interval boundaries become flatter and the time-discrete signal level of an interval is not already attained at the boundary of the interval but only after a certain “stabilization time”. Only after the level has been attained is the time-discrete analog signal meaningful, which is why this segment of the interval is designated the information-bearing segment of the time-discrete analog signal.
Image sensors which are read out serially supply such a time-discrete analog signal, for example. For instance in the case of an image sensor with a row of 512 pixels, the 512 pixels are read out in sequence, under the control of a clock signal, resulting in a time-discrete analog signal at the output of the image sensor which represents an illumination value of the first pixel in its first interval and an illumination value of the 512th pixel in its last interval. Obviously such a sensor could have a plurality of outputs and the readout, instead of being completely serial could be a mixture of serial and parallel. It can, however, be assumed that an image sensor will generally have fewer analog outputs than image points. This means that the analog values of a plurality of image points must be transmitted at the highest possible rate via the outputs of the image sensor using a multiplexing method. This results in the described time-discrete analog signal, which can also be viewed as a continuous signal composed of stepwise constant signal parts.
Normally conventional amplifier circuits, such as continuously operated source-follower or emitter-follower circuits, are employed to process such time-discrete analog signals or to drive an external and/or internal load capacitance. Such source-follower or emitter-follower amplifier stages are based on transistors with a signal-dependent dynamic operating point (AB operation). A disadvantage of such a continuously operated source follower or emitter follower is the fact that the magnitude of the output current for an output current direction is limited by the “predetermined” cross current, i.e. in AB operation by the cross current predetermined by the signal, through the transistor stage. If the specified cross current were simply to be increased, so as to make the stage faster, the power loss of the amplifier stage would become unacceptably high.
JP 62-220010 discloses a circuit for selectively lowering the base potential of a second transistor whose base is connected to an output node of a first transistor. Between the output node of the first transistor and a ground potential there is a current source. The first transistor, an npn transistor, has its emitter terminal connected to the output node and its collector terminal to a battery voltage, and an input potential can be applied to its base terminal.
JP-07245536-A discloses an amplifier circuit with an output control function which employs an inverted Darlington circuit. The Darlington circuit consists of a first transistor and a second transistor, the emitter terminal of the first transistor being connected to the collector terminal of the second transistor via a switch. Furthermore, the collector terminal of the first transistor is connected via a voltage supply, which is in turn connected to the emitter terminal of the second transistor. The input terminal of the circuit is formed by the base terminal of the first transistor while the output terminal of the circuit is formed by the collector terminal of the second transistor.